This invention relates to electrooptic waveguide circuits and, more particularly, to methods and apparatus for coupling energy between electrooptic waveguide substrates.
Research on integrated optics has been going on for some years now, with particular emphasis on developing effective and efficient electrooptic waveguide switches/modulators. Such devices are very attractive for signal encoding, multiplexing, optical frequency shifting, signaling and, particularly, switching. There is a large volume of literature on the subject of electrooptic waveguide modulation. The following two references, which present tutorial reviews, form a good starting point. Waveguide Electrooptic Modulators, Rod C. Alferness, IEEE Transactions on Microwave Theory and Techniques, Vol MTT-30, No. 8, August 1982, pp 1121-1137; and Guided-Wave Devices for Optical Communication, Rod C. Alferness IEEE Journal of Quantum Electronics, Vol. QE-17, No. 6, June 1981, pp 946-959.
A brief description of relevant portions of these articles is presented herein in conjunction with FIG. 1 for the sake of competeness.
Integrated waveguide modulators of a form not unlike the one shown in FIG. 1 can be created in various materials and accordance with various techniques. A waveguide modulator can be realized, for example, with light channels created in a Lithium Niobate substrate, such as channels 10 and 15 in FIG. 1. The channels may be created by diffusing Ti into the substrate, thereby causing the refractive index in the channels to be greater than in the surrounding region. When the difference in refractive indices is large enough, light that is injected into channel 10 at point 11 travels through the channel without significant dispersion into region 20. Somewhere along its path, the light traveling in channels 10 and 15 passes through region 30 of the substrate. Within region 30, channels 10 and 15 are situated in close physical proximity to each other and are bordered by electrodes 41 and 42. By choosing an appropriate separation between the waveguides and the interaction length, all of the light incident on one of the waveguides exits in the other via distributed evanescent coupling. Thus, the light traveling through channel 10 exits the substrate at point 17. In a similar fashion, light may be injected into channel 15 at point 16. It exits at point 12. Applying a voltage to the electrodes causes a phase shift in the light traveling through channel 10 within region 30. The phase shift reduces the coherent coupling between waveguides 10 and 15. With an appropriate voltage the output optical signal at port 17 reduces essentially to zero. The same conditions apply to light injected into channel 15.
The arrangement depicted in FIG. 1 is akin in its operation to a cross-connected double-pole, double-throw switch. When a voltage is applied to electrodes 41 and 42, light injected into ports 11 and 16 exits at ports 12 and 17, respectively. This is the "pass thru" connection. When no voltage is applied, the light of channels 10 and 15 crosses over to the other channels and exits at ports 17 and 12, respectively. This is the "crossover" connection. Viewed another way, when a detector is placed at one of the outputs and different signals are applied at the two input ports, then the FIG. 1 circuit serves as a selector, combiner, or multiplexer. Conversely, when the a detector is placed at both outputs, and only one input has an applied input signal, then the FIG. 1 circuit serves as a demultiplexer. Thus, the modulator of FIG. 1 can be a switch, a multiplexer, or a demultiplexer.
Implementing one switch, or even realizing a number of switches on a single substrate is only of marginal value. The key to utilization of the technology must lie in the ability to easily and effectively construct relatively large switching networks. To achieve that, three issues must be addressed. The first issue is the ability to create many switches on a single substrate. The second issue is the creation of an efficient architecture for building an optical switch network from individual switches and/or from smaller switch networks. The third issue is the development of means for interconnecting the substrates that contain the switches and the switch networks.
The latter two issues need better solutions than the ones that are available in the prior art.
Transmission of information from one electronic circuit board to another via an optical medium has been accomplished with fibers. This is often referred to as the optical backplane approach. Generally, this approach involves the use of light emitters and light detectors at the transmitting and the receiving ends of the transmission medium. It is an inefficient approach for interconnecting boards or substrates of electrooptic waveguides that contain a modest amount of logic (e.g., switches) per connection.
In the optical computing field, 3D interconnections are implemented with fiber arrays (e.g., U.S. Pat. No. 3,872,293 issued to E. L. Breen on Mar. 18, 1975), or in "free space". In "free space", the connection pattern is fixed since the optical signal paths are controlled in bulk by the medium. That, per se, is not an insurmountable impediment, and U.S. Pat. No. 4,913,959, issued on 06/05/1990 demonstrates that fact. That application discloses an arrangement for performing a perfect shuffle on an incoming set of signals. The perfect shuffle permits an arbitrary interconnection to be realized by the use of a sufficient number of interconnection stages.
Alas, the connection techniques that are used in free space optical computing are not applicable to waveguide electrooptics because these techniques basically rely on emission of light by the sending elements and detection of light by the receiving element. In a sense, this is the same technique that is used in the optical backplane. No specific match in polarization is generally necessary for the detection function.
Interconnection in three dimensional space has also been employed in the electronics arts (in contrast to optics), although in a different manner. For example, in "Compact Layout of Banyan/FFT Networks" Proc. CMU Conf. VLSI syst. Computations, 1982, pp 186-195, D. S. Wise described an arrangement whereby a crossover network can be decomposed in a manner that permits realization of the overall circuit from a collection of circuit boards that are stacked and interconnected in three dimensions. Of course, the spatial arrangement described by Wise is useful only for the particular class of networks that he described; but once he decided that such a spatial organization is useful, no major impediments were presented for implementing the desired electrical interconnection. Electronic circuit wire connections for the described arrangement are quite conventional.
That is not the case with integrated optics where light travels in controlled channels as described above. The polarization state of the light is very critical and the positioning alignment of the elements that emit light and the elements that accept the light is exacting.